Vascular compliance and mean circulatory filling pressure in trout: effects of ACE inhibition

Mean circulatory filling pressure (MCFP), whole body vascular compliance (C), and unstressed blood volume (USV) are important indexes of cardiovascular function in mammals, but they have not been measured in fish. In the present experiments, dorsal aortic (PDA) and sinus venosus (PSV) pressures were measured in unanesthetized trout before and during electrical cardiac fibrillation, while blood volume (BV) was manipulated between 50 and 150% of normal. Measurements were repeated after angiotensin-converting enzyme (ACE) inhibition with lisinopril. Cardiac fibrillation (zero-flow condition) rapidly (approximately 5 s) dropped PDA and increased PSV (equals MCFP). MCFP in normovolemic trout (4.8 +/- 0.3 mmHg) varied directly with BV. C determined from in vivo capacitance curves was similar to that obtained gravimetrically, in vitro (3.4 and 3.5 ml.mmHg-1.kg body wt-1, respectively). USV was 13.3 ml/kg body wt (approximately 45% of BV). ACE inhibition reduced PDA in unfibrillated trout at all BV and reduced PDA in fibrillated fish at BV > or = 80%. ACE inhibition did not affect PSV, MCFP, C, or USV. The systemic arteriovenous pressure gradient at zero flow (delta PF0) was greatest at 100% BV (8.2 +/- 0.5 mmHg) and was reduced by ACE inhibition at 80-120% BV. These results show that key indexes of venous function are readily measured in fish and that the trout venous system is not an effector of angiotensin-mediated regulation of arterial blood pressure. Thus angiotensin acts solely on arterial resistance vessels. Furthermore, the drop in delta PF0 during ACE inhibition is due to a decrease in arteriolar resistance.